Batteries can be used in various applications, such as electric vehicles and hybrid electric vehicles. The workable voltage of a single cell in a battery may be approximately 2˜4 volts, but some systems such as the electric vehicles and the hybrid electric vehicles may require higher voltages, e.g., 40 volts. Multiple cells can be coupled to each other in series to drive the electric vehicles and the hybrid electric vehicles.
In battery management, the status of cells, such as cell voltages, may be detected and measured by a measurement block, e.g., an analog-to-digital converter (ADC). The ADC can be implemented by devices having relatively low voltages. A voltage level shifter can be coupled between the cells and the ADC to shift the terminal voltages of each cell to lower voltages, e.g., from 40 volts to 2 volts, and the ADC detects and measures the cell voltages according to the shifted voltages.
FIG. 1 shows a conventional cell voltage detection circuit 100. The circuit 100 includes a battery pack 110, a cell voltage level shifter 120, and a detection and measurement block 140. The battery pack 110 includes battery cells 111-115 as shown in the example of FIG. 1. The level shifter 120 selects a battery cell according to a cell selection signal 180, and shifts the terminal voltages of the selected cell to lower voltages. In such way, the detection and measurement block 140 can measure the cell voltages for the cells 111-115, and output the measured results accordingly.
The level shifter 120 consumes current from the battery pack 110. By way of example, when the cell 112 is selected, a current IVH1 flows from the node H1 which is the positive terminal of the cell 112, through the level shifter 120, the cells 115-112, and back to the node H1. Moreover, a current IVL1 flows from the node L1 which is the negative terminal of the cell 112, through the level shifter 120, the cells 115-113, and back to the node L1. Therefore, the capacities of the cells 113-115 are degraded by the currents IVH1 and IVL1, while the capacity of the cell 112 is degraded by the current IVH1. Similarly, the level shifter 120 consumes current when measuring cell voltages for other cells, e.g., cells 111 and 113-115. As a result, the capacities of the cells 111-115 are unbalanced and the cell 115 located at the bottom of the battery 110 may have less capacity compared to other cells 111-114, thus affecting the available capacity of the battery pack 110.